We recently upgrade to ArcGIS Enterprise 11.2 With the new licensing level we have 25 creator named user licenses but only 17 Advanced Editor Add-on licenses.
We need to add advanced edit capabilities to some more creators.
I don't see a way to purchase more Advanced Editor Add-on licenses on the web site. I can purchase create, professional and professional plus but not Advanced Editor.
https://www.esri.com/en-us/arcgis/products/arcgis-enterprise/pricing#accordion-1-3
I would recommend that you contact your Esri Account Manager. If you do not know who that is, you can always contact Sales: https://www.esri.com/en-us/contact#c=us&t=1 and they should be able to get you taken care of.